392 ANIMAL PHYSIOLOGY. 
Foreign Gases and Respiration. These are divided into: 
1. Indifferent gases, as N, H, CH, which, though not in 
themselves injurious, are entirely useless to the economy. 
2. Poisonous gases, fatal, no matter how abundant the nor- 
mal respiratory food may be. They are divisible into: (a) those 
that kill by displacing oxygen, as NO, CO, HCN;; (b) narcotic 
gases, as CO,, N.O, producing asphyxia when present in large 
quantities; (c) reducing gases, as H.S, (NH,).8, PHs, AsHs, C.N:, 
which rob the hemoglobin of its oxygen. 
There are probably a number of poisonous products, some 
-of them possibly gases, produced by the tissues themselves and 
‘eliminated normally by the respiratory tract; and these are 
doubtless greatly augmented, either in number or quantity, or 
both, when other excreting organs are disordered. 
RESPIRATION IN THE TISSUES. 
We first direct attention to certain striking facts: 
1. An isolated (frog’s) muscle will continue to contract for 
a considerable period and to exhale carbon dioxide in the total 
absence of oxygen, as in an atmosphere of hydrogen; though, 
of course, there is a limit to this, and a muscle to which either 
no blood flows, or only venous blood, soon shows signs of 
fatigue. 2. In a frog, in which physiological saline solution 
has been substituted for blood, the metabolism will continue, 
carbonic anhydride being exhaled as usual. 3. Substances, 
which are readily oxidized, when introduced into the blood of 
a living animal or into that blood when withdrawn undergo 
but little oxidative change. 4. An entire frog will respire car- 
bonic dioxide for hours in an atmosphere of nitrogen. - 
Such facts as these seem to teach certain lessons clearly. It 
is evident, first of all, that the oxidative processes that give rise 
to carbon dioxide occur chiefly in the disswes and not in the 
blood; that in the case of muscle the oxygen that is used is first 
laid by, banked as it were against a time of need, in the form of 
intra-molecular oxygen, which is again set free in the form of 
carbon dioxide, but by what series of changes we are quite un- 
able to say. Though our knowledge of the respiratory processes 
of muscle is greater than for any other tissue, there seems to 
be no reason to believe that they are essentially different else- 
where. Theadvantages of this banking of oxygen are, of course, 
obvious; were it otherwise, the life of every cell must be at the 
mercy of the slightest interruption of the flow of blood, the 
